Automatic frequency control for receiver-transmitter



June 30, 1953 c. E. SHARP 2,644,077

AUTOMATIC FREQUENCY CONTROL FOR RECEIVER-TRANSMITTER Filed Dec. 6, 1945 R-F POWER FREQUENCY REACTANCE P OSCILLATOR TUBE INPUT AMPLIFIER MuLTIPLIER MQDULATOR l6 l4 I2 Io AUDIO ao- FILTER [20 (22 [as 2e\ R-F I-F AMP. F-M

- MIxER & AMPLIFIER LIMITER DETECTOR LocAL OSCILLATOR -24 AUDIO AMPLIFIER FIG. I.

TO TANK CIRCUIT IN VEN TOR.

HQ CHESTER E. SHARP BY Gym-4M. 2. M

ATTORNEY Patented June 30, 1953 f I. f '7 AUTOMATIC FREQUENCY CONTROL FOR RECEIVER-TRANSMITTER Chester E. 'Sharp, Little Silver, N. J., assignor to the United States of America as represented by the Secretary of War f Application December 6, 1945, Serial No. 633,254

p 1 Claim. (01.250-13) (Granted under Title 35, U. S. Code (1952),

frequency of a combined transmitter and receiver of the frequency modulation type.

A further object of my invention is 'toprovide a crystal-controlled frequency modulation system in which a single crystal stabilizes both the tuning of the receiver andthe center, or resting, frequency of the transmitter. V

. In accordance with a preferred embodiment of my invention, the transmitter is frequency mod'u-' lated by means of a reactance tube'wliich varies sec. 266) a cludes a radio frequency amplifier 20, a frequency changer, including a mixer 22'and a crystal-controlled local oscillator 24, an intermediate frequency amplifier and amplitude limiter 2B, and a frequency modulation detector 28, which demodulates the received Signal and yields an audio frequency output which may be amplified by an audio frequency amplifier and then translated by means of a sound reproducer or the like.

In frequency modulation systems wherein the master oscillator is frequency modulated by variation of the reactance of the tank circuit thereof, as is the case with the circuit above described, it is necessary to use an oscillator which is not stiff, so that its frequency can be readily modulated. Such oscillators are, however, inherently unstable in frequency so that the center frequency thereof is subject to considerable drifting.

To overcome this objection, it is a feature of my the operating frequency of a master oscillator. 2e invention touse the output of detector 28 to The receiver is preferably of the superheterodyne stabilize the center frequency of oscillator 12. type in which a piezoelectric crystal is used to As is well known in the art, the frequency modustabilize the frequency of the local oscillator. lation detector 28 is usually so tuned that it pro- The output of the frequency-modulation detector, vides a zero output at the center, or resting,*freor frequency discriminator, provides the audio quency of the incoming signal, said output varying signal as well as a frequency-dependent potential in magnitude as a function of the deviation of the which is fed to the reactance tube to control the signal frequency from said center frequency, and center frequency of the transmitter. varying in polarity as a function of the direction 'For a better understanding of the invention, of said deviation. The intermediate frequency together with'other and further objects thereof, 30 applied to detector 28 is determined by the difreference is had to the following description'taken ference between the frequencies of the transconnection with the accompanying drawing, mitted output and the local oscillator 24." Since wherein like parts are indicatedby like reference the local oscillator 24 is 'crystal' controlled, its numerals and wherein: frequency is stable. Hence, only variations in the Figure -1 is a block diagram of myinvention; center frequency of master oscillator l2 will and r 7 cause a change in the center frequency of the Figure 2 is a schematic circuit diagram of a signal applied to detector 28, and will result in portion of Fig. 1. g the generation of an error voltage in the output --'Referring to Fig. 1, there is shown a'frequency of detector 28. This voltage, which maybe ammodulated transmitter including a reactance tube 40 pl fied by a direct current a p fi if ss y, circuit H! which is coupled to the tank circuit of is applied, through a filter 30,-which keeps out -a master oscillator l2. Audio signals, such as the audio currents, to a control grid of reactance those derived from a microphone circuit, are imtube modulator ID. 1 The error voltage impressed pressed upon a control grid of the reactance tube on said reactance tube modulator I0 is of-such to vary-the effective reactance thereof, whereby magnitude and polarity that it will vary the frethe frequency of the oscillatorl2 is deviated to an quency of oscillator H! to such an extent and in extent proportional to the modulation voltage. such direction that the center frequency applied The frequencymodulated output of oscillator 12 to the input of detector Ill will be changedtc a is then applied to a buffer amplifier and frequency point where the output of said detector is subdoubler I4 and a radio frequency power amplifier 5O stantially zero, at which point the master oscil- IB, and then radiated through an antenna l8, lator is operating at the proper Center r q cy. Signals received on the antenna as well as the Reference is now made to Fig. 2 for a more transmitted signals, are impressed upon a fredetailed description of the frequency modulation quency modulation receiver, which in this case detector 28 and the reactance tube modulator l0 is of the superheterodyne type. The receiver inand the interconnection thereof. Thefrequency secondary, whereby both diode anodes are also fed in like phase. A connection is also mad e from said center tap to the junction of two load resistors 58 and 60 connected in series in the cathode circuits of said diodes. 'A radio-frequency bypass condenser 62 shunts said resistors. One end of said series-connected resistors is grounded. The other end thereof is coupled through a blocking condenser 64 to the audio amplifier circuits of the receiver.

The circuit thus far described is a well known the diode anodes are fedin like phase by one 'potential derived from the transformer primary through condenser 55. Said anodes are also fed in-opposite phase by a voltage which, because of the tuned transformer action, is 90 phase displaced with respect to said first voltage. At the exact center frequency the vector sum of the voltages applied to the diodes are equal, whereby the output voltages across resistors 58 and 69 oppose and cancel each other. For frequencies displaced from the center frequency the vector sum of the voltages applied to the diodes are unequal, whereby the resultant output voltage across the resistors 58 and 653 is proportional to the amount of said displacement, and the polarity of said output voltage is dependent-upon the direction of the frequency displacement.

A portion of the output of the frequency modulation detector 28 is also applied, through audio-frequency eliminating filter 30, comprising resistor 66 and condenser 68, to the grid of the triode section of tube 54, which acts as a direct current amplifier. The amplified voltage ap-- pears across the plate load resistor "H! in series with plate potential source H. The voltage across resistor 70 controls the grid bias of re actance tube l2 of the modulator Iii. r

Tube 12 functions as an electronic reactan'ce tube by virtue of the fact that 'a portion of the tank circuit voltage of the master oscillator I2 is shifted 90 by the series-connected resistor 14 and condenser 16. Hence, the current injected into the tank circuit by the plate circuit of tube .12 is phase displaced 90 with respect to the tank circuit voltage, whereby the plate circuit of the tube functions as a reactance. Variation of the voltage on the grid of the tube 12 will; therefore, vary the .efiective reactance across the tank circuit, and hence the operating frequency of the master oscillator.

Thegrid voltage of tube 12 is varied in two ways. One way is by applying a portion of the output of the frequency modulation detector, as above described. The other way is by means of the audio signal from the secondary of transformer l8, applied between the grid and cathode of-the tube through a predistorting filter, comprising resistor 80 and condenser 82, which emphasizes the high audio frequencies.

Resistors 80 and as functions asvoltage dividing resistors for the audio signals. Resistor 86 keeps radio frequency currents out of the audio transformer circuit. Condenser '88 is a blocking condenser. r

4 The manner in which the center frequency of master oscillator I2 is kept constant will now be described in greater detail with reference to Fig.

2. If said center frequency is exactly at the proper value, no resultant voltage is developed across the series-connected load, resistors 58 and 60. However, if said cente'r frequency drifts either above or below its normal value, a resultant voltage appears across said load resistors,

this voltage being positive when the frequency is lower and negative when the frequency is higher. A portion of this resultant voltage is applied to the grid of the triode' section of tube 54.

When the center frequency is normal, no voltage appears on the grid of tube 54. Under this condition, and because said grid is operated at zero bias, plate current will flow as follows: From the triode plate through resistor 70, and through battery H to ground and return to the filament of tube 54. The direction of this flow is such as torender the griclof reactance tube 12 negative at thenormalloperating grid voltage. I If the center frequency driftsto a higher 'value, the grid of tube 12 is rendered less negative, causing an increase in flow of plate current and, as a result, the center frequency is lowered and restored to its normal value. I V 7 Thereverse occurs when-the center frequency drifts to a lower value This causesthe grid. to tube 12 to be renderedumore :negative and results in an increase of the center frequency to its normal value.

I Because of the large time constant offilter 30, the demodulated audio signals are kept out-of the reactance modulator circuit 10, thus prevent-Q ing degenerative. feedback. 7 From the abovedescription it will-be seen that my invention provides a simple. means for stabilizing the center frequency of a frequencymodulatedtransmitter and receiver by the use of only a single frequency-stabilizing element, in this case a piezoelectric crystal. My invention also. provides a side-tone in the output of the receiver when the transmitter is operating on the proper frequency, thereby indicating at all times when the transmitter and receiver are operating properly. a

While there has beendescribed what is at present considered. a preferred. embodiment of my invention, it will be obvious to those skilled in the art that variouschanges and modifica tions may be made therein without departing from the invention; and it is aimed. in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention. r

Iclaim: r In combination, a frequency modulationtransmitter and receiver both connecteddn parallel to a common. antenna and both simultaneously operative,;said transmitter comprising. an oscillator, a reactance tube [circuitfor modulating the frequency of said oscillator; saidreceiver comprising a stable local oscillator,amixer an intermediate frequencyamplifier tuned tothe frequency; difference of said oscillators, a' frequency variationndetector for demodulating the output of said amplifier, and a transducer; for transforming the outputof said detector into sensory signals; and means including a signal eliminating filter coupled between said detector and said reactancetube tomaintain said frequency difierence substantially constant.= 1

' CHESTER SHARP.

. (References on following: page) i V Number a 5 References Cited in the file of this patent UNITED STATES PATENTS Name Date Herold Nov. 9, 1943 5 Ziegler et a1. June 26, 1945 Wolf et a1. June 4, 1946 Vogel Oct. 8, 1946 FOREIGN PATENTS Number Country Date OTHER REFERENCES Signal Corps Laboratories Annual Report for Fiscal Year 1937 on Means of Detecting the Approach of Aircraft and Marine Surface Craft Beyond Visual Range (pages 10 and 11).

Great Britain Feb. 24, 1 943 

